Normally, line pipes are welded and connected to one after another on the site; therefore, the pipe needs to be superior in dimensional precision in its pipe end, in particular, in its inner diameter. Moreover, normally, oil pipes are subjected to thread cutting processes in the end thereof, and these are joined to one another by tightening the ends. In this case also, the pipe needs to be superior in dimensional precision in its pipe end.
A known method for improving the inner diameter precision of a pipe end is that the pipe end is being expanded and sized by using an expanding apparatus.
FIG. 4 is a schematic drawing that explains a sizing method for the pipe end by using a conventional expanding apparatus. In the conventional method sizing for the pipe end, first, as shown in FIG. 4(a), with a pipe 1 to be sized being clamped by a chuck 2, a plug 5, connected to a cylinder 4, is moved in a direction indicated by an arrow in the Figure. Next, as shown in FIG. 4(b), the plug 5 is shoved to a predetermined position into the end of the pipe 1 so that the inner diameter of the pipe end is sized. Thereafter, as shown in FIG. 4(c), the plug 5 is moved in a direction indicated by another arrow in the Figure, and drawn from the pipe 1.
In this case, the plug to be used in the conventional expanding apparatus has a circular cross section, and is constituted by a taper portion and a diameter equivalent portion. The taper portion is a portion whose diameter gradually expands from the tip of the plug in the axial direction toward the rear end (from the left end to the right end in the Figure), and the diameter equivalent portion is a portion whose diameter is not varied. Here, the taper angle of the taper portion is kept constant.
Patent Document 1: JP2001-113329A